Fig. 4. (a) Polarization curves of Co_0.6Fe_0.4P and control samples for the HER in 1.0 M KOH (inset: the column diagram of HER activities for all Co_0.6Fe_0.4P catalysts); (b) the corresponding Tafel plots of all the Co_0.6Fe_0.4P catalysts and 20% Pt/C; (c) Nyquist plots of all Co_0.6Fe_0.4P catalysts at –0.15 V over the frequency range from 100 000 to 0.01 Hz.